The present invention relates to magnetoresistive read (MR) sensors for reading signals recorded in a magnetic medium, and more particularly, this invention relates to improving a free layer of MR sensors for improving operating characteristics.
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect (SV effect). In a spin valve sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO, FeMn, PtMn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In spin valve sensors, the spin valve effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the spin valve sensor and a corresponding change in the sensed current or voltage.
FIG. 1 shows a typical spin valve sensor 100 (not drawn to scale) comprising end regions 104 and 106 separated by a central region 102. The central region 102 has defined edges and the end regions are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an antiferromagnetic (AFM) layer 125. An underlayer 126 is positioned below the AFM layer 125.
The underlayer 126, or seed layer, is any layer deposited to modify the crystallographic texture or grain size of the subsequent layers, and may not be needed depending on the substrate. A variety of oxide and/or metal materials have been employed to construct underlayer 126 for improving the properties of spin valve sensors. Often, the underlayer 126 may be formed of tantalum (Ta), zirconium (Zr), hafnium (Hf), or yttrium (Y). Ideally, such layer comprises NiFeCr in order to further improve operational characteristics.
Free layer 110, spacer 115, pinned layer 120, the AFM layer 125, and the underlayer 126 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current Is from a current source 160 to the MR sensor 100. Sensor 170 is connected to leads 140 and 145 senses the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk). IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the spin valve effect.
In use, the GMR effect depends on the angle between the magnetizations of the free and pinned layers. More specifically, the GMR effect is proportional to the cosine of the angle xcex2 between the magnetization vector of the pinned layer (MP) and the magnetization vector of the free layer (MF) (Note FIGS. 2A and 2B). In a spin valve sensor, the electron scattering and therefore the resistance is maximum when the magnetizations of the pinned and free layers are antiparallel, i.e., majority of the electrons are scattered as they try to cross the boundary between the MR layers. On the other hand, electron scattering and therefore the resistance is minimum when the magnetizations of the pinned and free layers are parallel; i.e., majority of electrons are not scattered as they try to cross the boundary between the MR layers.
In other words, there is a net change in resistance of a spin valve sensor between parallel and antiparallel magnetization orientations of the pinned and free layers. The GMR effect, i.e., the net change in resistance, exhibited by a typical prior art spin valve sensor is about 6% to 8%.
FIG. 2C illustrates an air bearing surface (ABS) view of an MR sensor 200 according to another prior art embodiment, namely with an AP-coupled ferromagnetic layer structure (not drawn to scale). The MR sensor 200 comprises a ferromagnetic layer structure 220 and remaining unillustrated lower layers 221 (i.e. pinned layer, etc.) separated from each other by a non-magnetic, electrically conducting spacer layer 230. The unillustrated lower layers 221 have been excluded for clarity purposes.
The ferromagnetic layer structure 220 comprises a first ferromagnetic layer 222 and a second ferromagnetic layer 226 separated from each other by an antiparallel coupling (APC) layer 224 that allows the two ferromagnetic layers 222, 226 to be strongly coupled together antiferromagnetically. In use, the ferromagnetic layers are deposited in the presence of an orienting magnetic field to set the preferred magnetizations of the layers perpendicular to the ABS.
The two ferromagnetic layers 222, 226 have their magnetization directions oriented antiparallel, as indicated by arrows (see arrow heads pointing out of and in to the plane of the paper, respectively). The antiparallel alignment of the magnetizations of the two ferromagnetic layers 222, 226 is due to an antiferromagnetic exchange coupling through the APC layer 224, formed of a ruthenium (Ru) film or the like. Spacer layer 230 may be formed of a copper (Cu) film. Lead layers 260 and 265 are deposited on the end regions of a cap layer 205 to provide electrical connections for the flow of the sensing current Is from a current source to the MR sensor 200 for reasons similar to those set forth during reference to FIG. 1.
While the sensor valve of FIG. 1 relies on the longitudinal biasing of the hard bias layers to pin the associated free layer, the MR sensor 200 of FIG. 2 relies upon an antiferromagnetic (AFM) layer 270 for such longitudinal biasing. In use, inner portions of the ferromagnetic layer structure 220 are free, while outer portions of the ferromagnetic layer structure 220 are pinned by the AFM layer 270, in a manner that will soon be set forth.
Various parameters are pertinent to the operation of the MR sensor 200. In particular, such parameters include an inner thickness of the first ferromagnetic layer 222, tA; an inner thickness of the second ferromagnetic layer 226, tB; an outer thickness of the first ferromagnetic layer 222, tC; and an outer thickness of the second ferromagnetic layer 226, tD.
While the foregoing parameters, tA, tB, tC, and tD are physical measurements, they may be used to calculate another less tangible value, namely the magnetic thickness of the free and pinned portions of the ferromagnetic layer structure 220. In particular, the magnetic thickness of the inner free portions of the first ferromagnetic layers 222 and the second ferromagnetic layer 226 is calculated by Equation #1.
xe2x80x83Inner Free Portion Magnetic Thickness=tAxe2x88x92tBxe2x80x83xe2x80x83Equation #1
Further, the magnetic thickness of the outer pinned portions of the first ferromagnetic layer 222 and the second ferromagnetic layer 226 is calculated by Equation #2.
Outer Pinned Portion Magnetic Thickness=tDxe2x88x92tCxe2x80x83xe2x80x83Equation #2
It should be noted that the foregoing magnetic thicknesses play an instrumental role in stabilizing the ferromagnetic layer structure 220. To accomplish this stabilization, 1) the outer pinned portions of the first and second ferromagnetic layers 222 and 226 must be pinned and 2) the inner free portions of the ferromagnetic layer structure 220 must be provided with a longitudinal bias that maintains it in a single domain state or, in other words, maintain the field in the inner free portions in a single direction.
To promote strong pinning, it is necessary for the outer pinned portion magnetic thickness (tDxe2x88x92tC) to be small (i.e. about 10A). Further, to effect the desired longitudinal bias, the outer pinned portion magnetic thickness (tDxe2x88x92tC) must be greater than the inner free portion magnetic thickness (tAxe2x88x92tB) in order to work in conjunction with the AFM layer 270 to provide a biasing field.
Unfortunately, attempting to minimize the outer pinned portion magnetic thickness makes it difficult to ensure that the outer pinned portion magnetic thickness (tDxe2x88x92tC) is greater than the inner free portion magnetic thickness (tAxe2x88x92tB).
There is thus a need for a MR sensor system and associated method of manufacturing the same which are capable of accomplishing both of these requirements effectively to provide an optimally stabilized free layer.
A magnetoresistive read (MR) sensor system and a method for fabricating the same are provided. Included are a spacer layer, and a first ferromagnetic layer positioned above the spacer layer. Also included is an antiparallel layer positioned above the first ferromagnetic layer. A second ferromagnetic layer is positioned above the antiparallel layer. Such first and second ferromagnetic layers are antiferromagnetically coupled.
Next provided is a pair of antiferromagnetic layers positioned above the second ferromagnetic layer for defining inner free portions and outer pinned portions of the first ferromagnetic layer and the second ferromagnetic layer. The inner free portion of the first ferromagnetic layer has a first thickness tA, the inner free portion of the second ferromagnetic layer has a second thickness tB, the outer pinned portion of the first ferromagnetic layer has a third thickness tC, and the outer pinned portion of the second ferromagnetic layer has a fourth thickness tD.
The third thickness tC of the first ferromagnetic layer and the fourth thickness tD of the second ferromagnetic layer are substantially equal to the enhance the pinning of the outer pinned portions of the ferromagnetic layers. As mentioned earlier, an outer pinned portion magnetic thickness is defined as (tDxe2x88x92tC), and must be minimized to enhance the pinning of the outer pinned portions of the ferromagnetic layers.
To accommodate this design, a pair of third ferromagnetic layers with a fifth thickness tE are positioned above the antiferromagnetic layers. This way, the magnetic thicknesses of the outer portions of the first and second ferromagnetic layers may be substantially equal while still providing a longitudinal bias. As mentioned earlier, the outer pinned portion magnetic thickness must be greater than the inner free portion magnetic thickness (tAxe2x88x92tB) in order to work in conjunction with the AFM layer to provide adequate longitudinal bias. For purposes of providing the longitudinal bias, the outer pinned portion magnetic thickness is now defined as (tE+tDxe2x88x92tC), which may easily be designed to be greater than the inner free portion magnetic thickness (tAxe2x88x92tB) by increasing fifth thickness tE.
The associated method involves depositing a spacer layer and a first ferromagnetic layer positioned above the spacer layer. Also deposited is an antiparallel layer positioned above the first ferromagnetic layer. Next, a second ferromagnetic layer is positioned above the antiparallel layer such that the first ferromagnetic layer and the second ferromagnetic layer are antiferromagnetically coupled. Thereafter, a pair of antiferromagnetic layers is positioned above the second ferromagnetic layer for defining inner free portions and outer pinned portions of the first ferromagnetic layer and the second ferromagnetic layer. A pair of third ferromagnetic layers is then positioned above the antiferromagnetic layers.